1. Field of the Invention
The invention relates to a method and a device for the thermochemical production of synthesis gas from energy sources containing carbon, in particular from biomass.
2. Description of the Related Art
Synthesis gas from biomass is the starting product for the future solar hydrogen economy, in which the hydrogen is delivered to the end customer by a pipeline. Owing to the high efficiency in the decentral conversion of hydrogen into electricity, a surplus of electricity is obtained for almost all end consumers. Electricity and heat thus have the same value in this heat-controlled economy. For this reason, the use of electrical energy to provide the enthalpy of reaction for the production of synthesis gas is economical. Since electricity is only tradeable to a limited extent in the event of an electricity surplus, solar electricity, for example from wind energy, must be converted by water electrolysis into hydrogen and oxygen. The oxygen is therefore available for thermochemical gasification. In order to manage the logistics in the provision of biomass from agriculture and forestry, a plant size of between 20 and 500 MW is optimal. The plants should also be able to operate at an elevated pressure of between 6 and 40 bar, so that the gas produced can be fed directly into the regional gas network.
Essentially three methods are known for the thermochemical production of synthesis gas from biomass.
For the low power range, predominantly fixed bed gasifiers are encountered in a number of variants. Fixed bed gasifiers are adapted for a consistently high quality of biomass and are not appropriate for the production of high-quality synthesis gas which is suitable for further processing to form hydrogen.
The entrained flow gasifier is suitable in particular for high powers above 1 GW, because the reactor size of the entrained flow gasifier is relatively small. For small plants, the entrained flow gasifier is uneconomical owing to the high equipment outlay. The entrained flow gasifier requires substantially dry biomass or intermediate products, because the entrained flow gasifier operates at high temperatures with pure oxygen. The ash melts vitreously and is not usable as inorganic fertiliser. This is problematic in view of fertilisers becoming more expensive and less available.
The fluidised bed reactor has its strengths in the medium industrial power range of from 1 MW to 1 GW. When dealing with fluidised bed reactors, distinction is made between autothermal and allothermal gasification. In the case of autothermal gasification, a part of the biomass in the fluidised bed reactor is burnt in order to sustain the endothermic reactions taking place. In the case of allothermal gasification, the heat required is introduced by heat transfer. This may, for example, be done using heating rods in the fluidised bed or using a circulating heat-exchange medium. Sand, which is heated in a second reactor by burning a part of the biomass, is mostly used as a heat-exchange medium. There is such a gasifier with a thermal power of 8 MW in Güssing, Austria. This plant was presented at the 1st International Ukrainian Conference on BIOMASS FOR ENERGY; Sep. 23-27, 2002, Kiev, Ukraine by M. Bolhar-Nordenkampf et al. under the title: “Scale-up of a 100 KWth pilot FICFB to 8 MWth FICFB-gasifier demonstration plant in Güssing (Austria)”. DE 10 2004 045772 A1 discloses a method with a circulating heat-exchange medium, which additionally uses the heat tonality in the conversion of CaO to CaCO3. The fluidised bed reactor is operated below the sintering temperature of the ash being formed, which makes the ash usable as an inorganic fertiliser.
Oxygen, air and steam are used as a fluidisation gas and oxidising agent for the carbon in the synthesis gas reactor with a fluidised bed. In allothermal gasification, generally only steam is used. Autothermal gasification is operated with air. Pure oxygen is used in mixtures with steam and air, because pure oxygen would lead to local overheating in the fluidised bed. The use of air leads to dilution of the synthesis gas with nitrogen and CO2, which makes exploitation for electricity generation and further processing to form products such as hydrogen, methane, methanol or liquid propellants difficult. The provision of steam requires additional outlay of energy and increases the investment costs.
According to the prior art, the biomass is fed directly into the fluidised bed of the synthesis gas reactor. In the fluidised bed, the pyrolysis to form primary tars and the final reaction to form synthesis gas take place simultaneously within a short time. The tar content in the synthesis gas is therefore very high. The tar has to be removed by elaborate methods. Tar build-ups in apparatus furthermore often lead to failure of the entire plant.
Among all the known gasification methods, gasification in a fluidised bed reactor is distinguished in that the ash is not melted and can therefore be used as inorganic fertiliser in agriculture. The synthesis gas obtained does, however, have a high tar content. This is a great disadvantage for using the synthesis gas.